A modern electronic ballast is optimized for maximum electrical efficiency by minimizing consumption of electrical power. Unfortunately, the changes made to improve the efficiency of the ballast reduce certain internal electrical resistances. This gives rise to a new problem: the occurrence of a large inrush of current upon the first application of electrical power. This inrush current flows as the main filter capacitor in the ballast charges to its steady state value. For a lighting circuit that contains a multiplicity of ballasts, the combined magnitude of the inrush is potentially large enough to cause contact failure of the switching device due to arcing and contact welding.
An example of such a lighting circuit is one in which certain lights are controlled by a sensor that detects the presence of personnel by sensing their motion. With conventional technology, switch device failure is common.
The present invention presents a simple, cost-effective circuit for limiting inrush current to the ballast. The inventive circuit contains a current-limiting resistor that is active for a brief interval during startup. This resistor is subsequently bypassed from the principal current path by a switching transistor. The circuit of this invention provides minimal power loss during operation.